"If you want to hang around with me, fine, but I'm going to do what I want and don't want to hear any complaints. If you don't like it, the door is right there."

The way he "won't let" you see anyone else but requires you to "let" him do so... The way he has Cathy the Spare Tire™ waiting in the wings while he decides whether your relationship is worth continuing... Those two things tell me that he wants to break up with you but doesn't have the maturity to do so himself so he wants to drive you to the end of your rope so that YOU will leave and he will not have to grow up.

I think you should oblige him. I don't know how financially and logistically intertwined your lives are, but if you can't pack your bags and move out, and if you can't afford to pack his bags and kick him out, the very least you should do is corner him and say that YOU have decided that since HE gets to see other people, YOU can do the same. No negotiation. You don't have to actually go GET another relationship, just assert some equality/fairness in THIS one.

And Ceoli is right. Fuck Cathy and her sniping. She is LESS than nobody to you because you reached out to her and she basically slapped your hand away.